Occasionally when I'm feeling the need to tune up my software development skills. I'll venture off in to unfamiliar territory in hopes that I can pick up a few things that I can use later on.

Lisp has always been an elusive language for me. I can appreciate how cool it is to those who are fans of it, yet I have not gotten to the point where I'm productive in it. Nonetheless, occasionally I will try to make it through a tutorial, hoping to get a better understanding of it.There's a decent book online for it and this chapter is a mind bender(for me anyway)
Practical Common Lisp: A Simple Database

Mainly because it shows you what's cool about lisp. Which is the macro functionality which allows you to easily(relatively) create your own Domain Specific Languages(DSL's)